A typical circuit board manufacturing process includes a soldering stage and a cleaning stage. One type of circuit board manufacturing process involves soldering surface mount components to the surface of a bare circuit board and then cleaning the circuit board to remove contaminants. In the soldering stage of this process, automated equipment prints solder paste over mounting locations of the circuit board, places the surface mount components over the mounting locations and in contact with the printed solder paste, and applies heat to activate flux and to melt solder within the solder paste. As a result, solder joints form between the components and the surface of the circuit board. This process may be repeated on the opposite surface of the circuit board in order to utilize space on both sides of the circuit board.
In the cleaning stage, cleaning equipment washes and rinses the populated circuit board to remove contaminants (e.g., organic compounds, cleaning residues, etc.). If the contaminants are not removed, the contaminants could create an undesirable electrical leakage path between conductive structures on the circuit board. In particular, over time, the contaminants could absorb moisture from the air and operate as a catalyst that deposits material between neighboring conductive surfaces eventually resulting in an electrical short.
For example, suppose that a recently manufactured circuit board is placed into operation within an electronic system. Further suppose that an organic contaminant exists on the circuit board surface. Over time, this contaminant may produce a wet battery effect between neighboring circuit board conductors (e.g., between surface traces, solder joints, pads, vias, pins, etc.). That is, the contaminant may absorb moisture and the acidity of the contaminant, in combination with the electrical biasing of the circuit board conductors when the circuit board is in operation, may cause residual metallic ions to migrate toward particular conductive surfaces. Eventually, current leakage commences through a formed conductive path between the neighboring circuit board conductors. At that time, the leakage source is essentially the cathode and the end of the path is essentially the anode of a wet battery. If the conductive path carries a relatively small amount of current, the path may cause only minor damage (e.g., an intermittent short resulting in corrupted data, a product functional failure, etc.). However, if the path continues to grow, the path may carry a high amount current which causes a significant amount of damage (e.g., permanent damage to a component, a thermal event, etc.).
There are a variety of conventional approaches to cleaning circuit boards during the cleaning stage of the circuit board manufacturing process. Some conventional approaches involve the application of an aqueous solvent to the circuit board surfaces. The aqueous solvent typically has a surface tension which is lower than water thus enabling the solvent to more easily penetrate into small gaps and tight areas such as narrow regions between component packages and the circuit board surface (e.g., the narrow space between a Ball Grid Array device and the circuit board surface). Furthermore, the solvent typically includes a surfactant that saponifies contaminants residing on the populated circuit board. That is, the solvent acts as a saponifier by breaking down and dissolving contaminants which are common to the circuit board manufacturing process (e.g., organic residues, dirt, dust, grease, metal ions, etc.).
One conventional cleaning approach which uses aqueous solvent (hereinafter referred to as the “in-line spray approach”) involves placing circuit boards onto a conveyor assembly which individually conveys the circuit boards horizontally past an upper set of sprayers which sprays the solvent vertically downward onto the top surface of the circuit boards, and a lower set of sprayers which sprays the solvent vertically upward onto the bottom surface of the circuit boards. As the solvent hits the circuit board surfaces and the circuit board components, the solvent tends to puddle and drip. As the solvent falls off of the circuit boards, the solvent carries away contaminants. The conveyor assembly continues to convey the circuit boards individually past other upper and lower sets of sprayers which spray de-ionized water onto the circuit boards to remove any remaining aqueous solvent.
Another conventional cleaning approach which uses aqueous solvent (hereinafter referred to as the “in-line immersion approach”) involves spray under immersion. This approach involves placing circuit boards onto a conveyor assembly which individually conveys the circuit boards horizontally through a tray of aqueous solvent, e.g., the circuit boards are immersed in aqueous solvent which is two to four inches deep. While a circuit board is in the tray, sprayers above and below the circuit board spray the aqueous solvent onto the circuit board. The aqueous solvent dissolves and carries away contaminants residing on that circuit board. Then, as in the earlier-described in-line spray approach, the conveyor assembly individually conveys the circuit boards horizontally through a rinsing stage (e.g., through a tray of de-ionized water, or past upper and lower sets of sprayers which spray de-ionized water) to remove any remaining aqueous solvent.
Yet another convention cleaning approach which uses aqueous solvent (hereinafter referred to as the “batch cleaning approach”) involves a technician manually placing multiple circuit boards into an empty tub one at a time by hand so that the circuit boards are vertical and parallel to each other. The technician then closes a lid over the tub and activates a pump assembly which fills the tank with the aqueous solvent. When the tank is full and the circuit boards are completely covered with the solvent, a pair of nozzles on one side of the tank directs the solvent past the circuit boards. In particular, the aqueous solvent tends to flow in a single direction through the defined spaces between the parallel circuit boards. As the solvent flows by the circuit boards, the solvent dissolves and carries away contaminants from the circuit board surfaces. The technician then empties the tank of the solvent, and partially opens the lid so that an air duct outside the tub can draw away solvent vapors from the tub periphery. The technician then one by one manually transfers the circuit boards by hand onto a conveyor assembly that transfers the circuit boards individually through a rinsing stage to remove any remaining solvent.
It should be understood that there are a variety of conventional “No Clean” approaches which use “No Clean” fluxes and “No Clean” solder pastes. The suppliers of these “No Clean” fluxes and “No Clean” solder pastes contend that the use of such fluxes and solder pastes by the circuit board manufacturer alleviates the need to clean circuit boards using a cleaning process after the soldering process. Such fluxes and solder pastes are best suited for particular types of soldering metals (i.e., Lead as in Tin/Lead solder alloys) and at particular soldering temperatures.